The present invention relates powder of alloy containing a rare earth element Sm, a transition metal Fe, and nitrogen, and, more particularly, to such alloy powder having a spherical shape with superior magnetic properties.
In recent years, there have been ever-increasing demands for rare earth-transition metal based magnetic materials because of their superior magnetic properties, in spite of the fact that they are very expensive as compared with ferrite, etc. Among these, since Nd-based magnets have particularly high magnetic properties as compared with Sm-based magnets, and are inexpensive, they have come to be mainly used among rare earth magnets.
Here, Rxe2x80x94Fexe2x80x94N based alloys have been known, which are formed by nitriding Rxe2x80x94Fe based alloys that are rare earth-transition metal based magnetic materials. Magnets of this type have been developed extensively since they have superior characteristics, including possibility of higher coercive force as compared with Rxe2x80x94Fexe2x80x94B based materials resulting from high Curie Points not less than 150xc2x0 C., high stability with small temperature variations in magnetic properties and high weather resistance.
Rxe2x80x94Fexe2x80x94N based alloys are manufactured in the form of powder, molded into a desired shape in combination with a filler, and then utilized as bonded magnets. Although alloy powder of this type exerts a high anisotropic magnetic field, a molded magnet made from the alloy powder of this type is difficult to obtain a high coercive force. In order to obtain a high coercive force, the magnet needs to be finely pulverized, or utilized as a metal bonded magnet containing a metal like Zn as a binder. In the case of the finely pulverized magnet alloy, the particles are oxidized, or subjected to distortion or residual stress, resulting in degradation in other magnetic properties, e.g., a reduction in residual magnetization. In the case of the metal bonded magnet, this method is considerably expensive as compared with plastic binding used in normal bonded magnets, impossible to provide practical applications.
Magnetic alloy powder has an inherent mono-magnetic domain size, and it has been known that magnetic powder whose particle size is set closer to this mono-magnetic domain size can show a maximum coercive force. For magnetic materials containing a rare earth element and a transition metal, the mono-magnetic domain size is several micrometers. Therefore, to improve magnetic properties of alloy powder serving as a magnetic material, it is essential to provide a process for forming fine particles.
With respect to the process for a magnetic material containing a rare earth element and a transition metal, a reduction-diffusion method has been known in which a mixture of powders of a rare earth metal oxide and a transition metal with metal calcium is heated in an inert gas atmosphere so that the rare earth oxide is reduced to metal and transferred into the transition metal, to form an alloy (see Japanese Patent Publication Nos. JP-A61-295308, JP-A5-148517, JP-A5279714 and No. JP-A6-81010). This reduction-diffusion method is advantageous in that an inexpensive rare earth oxide may be used and alloued simultaneously with the reducing process. This method has been widely used in manufacturing an intermetallic compound SmCo5 or an Smxe2x80x94Co alloy used for permanent magnets. Moreover, in the case where the abovementioned Rxe2x80x94Fexe2x80x94N based alloy powder is manufactured, after the Rxe2x80x94Fe alloy has been reduced by this method, the reduced alloy is subjected to a nitrding process to form magnetic powder of an Rxe2x80x94Fexe2x80x94N based alloy.
In this reduction-diffusion method, an oxide of a rare earth element having a particle size of not more than several micrometers is used as a material, and the particle size of the magnetic powder obtained after reduction becomes smaller to a certain degree; however, this method is still not sufficient to provide fine magnetic powder corresponding to the mono-magnetic domain size. This is because the particle size of a material iron-based metal is quite large as compared with that of the rare earth element oxide. Therefore, conventionally, this reduced powder is nitrided, and then finely pulverized to the mono-magnetic domain size so as to exert a sufficient coercive force; thereafter, formed into a bonded magnet, whereas the resulting bonded magnet exhibits only a low residual magnetization.
For bonded magnets, when its magnet particles are provided as fine particles, its filling rate becomes low, resulting in a limitation in the density of the magnetic powder contained in its molded body. Moreover, when the bonded magnet is oriented toward a magnetic field applied, the distorted shape of the fine particles after pulverization makes it difficult to align the fine particles in a direction of easy magnetization axis toward the magnetic field, resulting in degradation in degree of alignment and degree of orientation.
An objective of the present invention is to provide powder of an Smxe2x80x94Fexe2x80x94N based alloy having high magnetic performances, in particular, a high coercive force by optimizing a particle size and shape of the alloy powder.
Another objective of the present invention is to provide a method for manufacturing powder of an Smxe2x80x94Fexe2x80x94N based alloy having high magnetic performances, in particular, a high coercive force, without the need for a mechanical method such as a finely pulverizing process.
In the present invention, Smxe2x80x94Fexe2x80x94N based alloy particles are finely divided to approximate a particle size to its mono-magnetic domain size or the vicinity thereof, and are simultaneously provided with a spherical shape, so that, when magnetizing bonded magnet toward a magnetic field, the fine particles can increase in degree of orientation in a direction of its easy magnetization, thereby increasing in coercive force.
In particular, in the Smxe2x80x94Fexe2x80x94N based magnetic powder of the present invention, the alloy powder is set to have an average particle size in the range of 0.5 to 10 xcexcm. Moreover, the magnetic powder is set to have an average degree of needle shape of not less than 75% in approximation of the spherical particles. In the present description, an average degree of needle shape is provided as an average of the degrees of needle shape of the individual particles which is defined by the following equation:
Degree of needle shape=(b/a)xc3x97100(%) 
where a represents the longest diameter on a projection image of a particle, and b represents the largest diameter vertical to the a of the particle. In particular, a shows the longest length on a particle image projected on a plane and b is the largest size vertical to the a on the same projection.
In addition to the average particle size in the range of 0.5 to 10 xcexcm, the Smxe2x80x94Fexe2x80x94N based magnetic powder of the present invention is set to have an average degree of roundness of not less than 78% as means for estimating the spherical particles. Here, the average degree of roundness is obtained as an average of values of roundness of the respective particles defined by the following equation:
Degree of roundness=(4nS/L)xc3x97100(%) 
Here, S and L represent an area of a particle projection and a peripheral length of the outline of the particle image, respectively, which are measured on the particle image projected on the plane.
The process for producing Smxe2x80x94Fexe2x80x94N based magnetic powder of the present invention uses a combination of reduction-diffusion technique of a metal oxide and nitriding technique, in which not less than half or all of the Fe source of a starting material for an Smxe2x80x94Fe based alloy is prepared as iron oxide, and a mixture of the iron oxide with samarium oxide is reduced by a metallic reducing agent such as Ca. Thus, alloy particles having a shape distribution close to spherical shape are obtained. The magnetic powder obtained by nitriding the alloy particles has a spherical shape or a similar shape to a spherical shape, allowing the particles to easily rotate in a magnetic field direction when they are magnetized in a resin bond. In this manner, the frequency of orientation of each magnetic particle toward the applied magnetic field is increased so that the magnetic particles in a bonded magnet are easily magnetized.
In the present invention, from the fact that in the reduction-diffusion method, the size of reduced particles is greatly dependent on the particle size of the material particles, oxide powder having fine particles may be used as starting material particles.
For this purpose, the present invention may preferably use a mixture of oxide particles of iron oxide and samarium oxide obtained through a co-precipitation method as a starting material. In other words, a precipitation of the mixture of iron oxide and samarium oxide is obtained by the co-precipitation method from a solution in the present of Fe and Sm co-existing, and decomposed and oxidized through calcination or another method to produce an oxide, and this oxide is available. In the present invention, the co-precipitation method and calcination achieve a high degree of a mixed state between Fe and Sm, and provide very fine oxide particles having a spherical shape; thus, the resulting magnetic powder is reduced and diffused which have a size and a degree of needle shape similar to those of the material oxide particles.
The method of the present invention further may include a process in which the oxide from the co-precipitation method is partially reduced preliminarily, so that the material powder, preliminarily reduced, can be more easily reduced and diffused by the metallic reducing agent such as Ca as described above. In the preliminary reduction, a gas reducing process using hydrogen, etc. may be used, and the resulting mixture, part of the oxide of which has been reduced, contains metal iron, iron oxide and samarium oxide, and is used for reduction and diffusion.